: Pneumocystis carinii (Pc) is a major cause of opportunistic infection and mortality in immunosuppressed individuals, particularly those with AIDS. Antifolates have been the clinically most effective anti-Pc agents to date but their use has been limited by problems of toxicity and resistance. Pc dihydrofolate reductase (DHFR) has been recently cloned. Its affinity for various antifolates indicates that antibacterials such as trimethoprim and pyrimethamine are affective inhibitors as are lipophilic antifolates such as the second generation anticancer agents trimetrexate and piritrexim. This, it is proposed, is due to specific interactions of Pc DHFR with these compounds. This hypothesis will be tested by comparing the crystal structures of human and Pc DHFRs in complex with the cofactor NADPH and several Pc- selective antifolates. The observed differences will be exploited for structure-based design of new antifolates for use as anti-AIDS adjuvants by the applicant together with her collaborators.